1. Field of Invention
The present invention relates to a light emitting device driver circuit; particularly, it relates to such light emitting device driver circuit with relatively fewer high voltage switches.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art light emitting diode (LED) driver circuit 10 and its related circuits. As shown in FIG. 1A, the LED driver circuit 10 includes a switch circuit 11, a switch control circuit 12, and a constant current source 13. The LED driver circuit 10 drives an LED circuit 20. The LED circuit 20 includes plural LEDs connected in series, e.g., LED1, LED2, LED3, and LED4 as shown in FIG. 1. The switch circuit 11 includes plural switches (e.g., four switches S1, S2, S3, and S4 as shown in FIG. 1), which are electrically connected to the corresponding LEDs LED1, LED2, LED3, and LED4 respectively. A rectifier circuit 30 rectifies an AC voltage from an AC power source 40, to generate a rectified input voltage Vin as shown in FIG. 1B. The LED driver circuit 10 drives the LED circuit 20 by respectively turning ON and OFF the switches S1-S4 according to the level of the rectified input voltage Vin, such that one or more of the LEDs LED1-LED4 glow accordingly.
For example, as shown by the signal waveforms in FIG. 1B, when the level of the rectified input voltage Vin is lower than level L1, the switch control circuit 12 turns OFF the switches S1-S4. When the level of the rectified input voltage Vin is between levels L1 and L2, the switch control circuit 12 turns ON the switch S1 and turns OFF the switches S2-S4, whereby the LED LED1 glows. Similarly, when the level of the rectified input voltage Vin is between levels L2 and L3, the switch S2 is turned ON, and the switches S1 and S3-S4 are turned OFF, whereby the LEDs LED1-LED2 glow. Similarly, when the level of the rectified input voltage Vin is between levels L3 and L4, the switch S3 is turned ON, and the switches S1-S2 and S4 are turned OFF, whereby the LEDs LED1-LED3 glow. Similarly, when the level of the rectified input voltage Vin exceeds level L4, the switch S4 is turned ON, and the switches S1-S3 are turned OFF, whereby the LEDs LED1-LED4 glow. U.S. Pat. No. 6,989,807, U.S. Pat. No. 7,081,722 and US 2011/0273102 are relevant prior art patents for reference.
In such prior art, the current source 13 provides a constant current, that is, when one or more of the LEDs LED1-LED4 glow, a current I1 flowing through the conductive LED(s) is the constant current. Referring to the signal waveform of the current I1 as shown in FIG. 1B, regardless how many of the LEDs LED1-LED4 glow, the current flowing through the conductive LED(s) is the constant current. Only when the level of the rectified input voltage Vin is lower than the level L1, i.e., when all the switches S1-S4 are turned OFF, the current I1 is zero current.
In comparison with a conventional driver circuit which drives the LED circuit with a DC voltage, an advantage of the prior art LED driver circuit 10 is that: the manufacturing cost of the LED driver circuit 10 is relatively lower because it does not need to convert the rectified input voltage to a DC voltage. If the rectified input voltage has a frequency which is enough high, naked eyes will not perceive any flicker of the LED circuit 20. However, a disadvantage of the prior art LED driver circuit 10 is that each LED requires to be connected to a corresponding high voltage switch, and each high voltage switch requires to be connected to a corresponding pin in the LED driver circuit 10; therefore, the size and manufacturing cost of the switch control circuit 12 are relatively high.
In view of above, the present invention proposes a light emitting device driver circuit with relatively fewer high voltage switches.